Process for preparing polyol esters

ABSTRACT

The present invention relates to a process for preparing polyol esters by reacting polyols with linear or branched aliphatic monocarboxylic acids having 3 to 20 carbon atoms by partial recycling of the aliphatic monocarboxylic acid removed into the esterification reaction or into subsequent esterification batches.

CLAIM FOR PRIORITY

This application is based on German Application No. 10 2009 048 772.7,entitled “Verfahren zur Herstellung von Polyolestern”, filed Oct. 8,2009, the priority of which is hereby claimed and the disclosure ofwhich is incorporated herein by reference.

TECHNICAL FIELD

The invention relates to a process for preparing polyol esters fromlinear or branched aliphatic monocarboxylic acids having 3 to 20 carbonatoms and polyols by converting the starting compounds with partialrecycling of the aliphatic monocarboxylic acids.

BACKGROUND OF INVENTION

Esters of polyhydric alcohols, also known as polyol esters, find avariety of uses on a large scale in industry, for example asplasticizers or lubricants. The selection of suitable starting materialsallows the physical properties, for example boiling point or viscosity,to be controlled, and the chemical properties, such as hydrolysisresistance or stability to oxidative degradation, to be taken intoaccount. Polyol esters can also be tailored to the solution of specificperformance problems. Detailed overviews of the use of polyol esters canbe found, for example, in Ullmann's Encyclopaedia of IndustrialChemistry, 5th edition, 1985, VCH Verlagsgesellschaft, Vol. A1, pages305-319; 1990, Vol. A15, pages 438-440, or in Kirk Othmer, Encyclopaediaof Chemical Technology, 3rd edition, John Wiley & Sons, 1978, Vol. 1,pages 778-787; 1981, Vol. 14, pages 496-498.

The use of polyol esters as lubricants is of great industrialsignificance, and they are used particularly for those fields of use inwhich mineral oil-based lubricants meet the requirements set onlyincompletely. Polyol esters are used especially as turbine engine andinstrument oils. Polyol esters for lubricant applications are basedfrequently on 1,3-propanediol, 1,3-butanediol, 1,4-butanediol,1,2-hexanediol, 1,6-hexanediol, neopentyl glycol, trimethylolpropane,pentaerythritol, 2,2,4-trimethylpentane-1,3-diol, glycerol or3(4),8(9)-dihydroxymethyltricyclo[5.2.1.0^(2,6)]decane, also known asTCD alcohol DM, as the alcohol component.

Polyol esters are also used to a considerable degree as plasticizers.Plasticizers find a variety of uses in plastics, coating materials,sealing materials and rubber articles. They interact physically withhigh molecular weight thermoplastic substances, without reactingchemically, preferably by virtue of their swelling and dissolutioncapacity. This forms a homogeneous system, the thermoplastic range ofwhich is shifted to lower temperatures compared to the originalpolymers, one result being that the mechanical properties thereof areoptimized, for example deformation capacity, elasticity and strength areincreased, and hardness is reduced.

In order to open up the widest possible fields of use to plasticizers,they must fulfill a series of criteria. They should ideally be odorless,colorless, and light-, cold- and heat-resistant. Moreover, it isexpected that they are insensitive to water, comparatively nonflammableand not very volatile, and are not harmful to health. Furthermore, theproduction of the plasticizers should be simple and, in order to meetecological requirements, avoid waste substances, such as by-productswhich cannot be utilized further and wastewaters comprising pollutants.A specific class of polyol esters (they are referred to as G esters forshort) contains diols or ether diols as the alcohol component, forexample ethylene glycol, diethylene glycol, triethylene glycol,tetraethylene glycol, 1,2-propylene glycol or higher propylene glycols.They can be prepared in different ways. In addition to the reaction ofalcohol and acid, optionally in the presence of acidic catalysts,further processes are employed in practice to obtain G esters, includingthe reaction of diol with acid halide, the trans-esterification of acarboxylic ester with a diol, and the addition of ethylene oxide ontocarboxylic acids (ethoxylation). In industrial manufacture, only thedirect reaction of diol and carboxylic acid and the ethoxylation ofcarboxylic acids have become established as production processes,preference usually being given to the esterification of diol and acid.This is because this process can be performed with no particularcomplexity in conventional chemical apparatus, and it affords chemicallyhomogeneous products. Compared to this, ethoxylation requires extensiveand costly technical equipment. Ethylene oxide is a very reactivechemical substance. It can polymerize explosively and forms explosivemixtures with air within very wide mixing ranges. Ethylene oxideirritates the eyes and respiratory pathways, leads to chemical burns andto liver and kidney damage, and is carcinogenic. The handling thereoftherefore entails extensive safety measures. Moreover, scrupulouscleanliness of storage apparatus and reaction apparatus has to beensured, in order to rule out the formation of undesired impurities as aresult of side reactions of the ethylene oxide with extraneoussubstances. Finally, the reaction with ethylene oxide is not veryselective, since it leads to mixtures of compounds of different chainlength.

The direct esterification of alcohols with carboxylic acids is one ofthe basic operations in organic chemistry. In order to increase thereaction rate, the conversion is typically performed in the presence ofcatalysts. The use of one of the reactants in excess and/or the removalof the water formed in the course of the reaction ensures that theequilibrium is shifted in accordance with the law of mass action to theside of the reaction product, i.e. of the ester, which means that highyields are achieved.

Comprehensive information regarding the preparation of esters ofpolyhydric alcohols, also including esters of ethylene glycols and fattyacids, and regarding the properties of selected representatives of thesecompound classes can be found in Goldsmith, Polyhydric Alcohol Esters ofFatty Acids, Chem. Rev. 33, 257 ff. (1943). For example, esters ofdiethylene glycol, of triethylene glycol and of polyethylene glycols areprepared at temperatures of 130 to 230° C. over reaction times of 2.5 to8 hours. To remove the water of reaction, carbon dioxide is used.Suitable catalysts mentioned for the esterification of polyhydricalcohols are inorganic acids, acidic salts, organic sulfonic acids,acetyl chloride, metals or amphoteric metal oxides. The water ofreaction is removed with the aid of an entraining agent, for exampletoluene or xylene, or by introducing inert gases such as carbon dioxideor nitrogen.

The production and the properties of fatty acid esters of thepolyethylene glycols are discussed by Johnson (edit.), Fatty Acids inIndustry (1989) Chapter 9, Polyoxyethylene Esters of Fatty Acids, and aseries of preparative hints are given. Higher diester concentrations areachieved by the increase in the molar ratio of carboxylic acid toglycol. Suitable measures for removing the water of reaction areazeotropic distillation in the presence of a water-immiscible solvent,heating while passing through an inert gas, or performing the reactionunder reduced pressure in the presence of a desiccant. When the additionof catalysts is dispensed with, longer reaction times and higherreaction temperatures are required.

Both reaction conditions can be made milder by the use of catalysts. Inaddition to sulfuric acid, organic acids such as p-toluenesulfonic acidand cation exchangers of the polystyrene type are the preferredcatalysts. The use of metal powders, such as tin or iron, is alsodescribed. According to the teaching from U.S. Pat. No. 2,628,249, colorproblems in the case of catalysis with sulfuric acid or sulfonic acidscan be alleviated when working in the presence of activated carbon.

A procedure in which esters of diethylene glycol and triethylene glycoland of caprylic acid are prepared without addition of catalyst is knownfrom U.S. Pat. No. 2,469,446. The esterification temperature is in therange from 270 to 275° C. and the water of reaction is driven out bymeans of a carbon dioxide stream.

The reaction regime in which the addition of a catalyst is dispensedwith generally works with a molar excess of the particular carboxylicacid, which, owing to its acidity, also acts as a catalyst.

For the removal of the water of reaction formed in the ester formationfrom the polyol and the carboxylic acids, various processes are known.For example, the water of reaction formed is distilled out of thereaction vessel together with the excess carboxylic acid and passed intoa downstream phase separator in which carboxylic acid and water separateaccording to the solubility properties thereof. In some cases, thecarboxylic acid used also forms an azeotrope with water under thereaction conditions and is capable of removing the water of reaction asan entraining agent. Also employed are azeotropic distillation in thepresence of an added water-immiscible solvent, heating of the reactionmixture while passing an inert gas through, and the reaction of thepolyol and carboxylic acid starting materials under reduced pressure orin the presence of a desiccant. Especially the removal of water byazeotropic distillation has been found to be useful for theestablishment of the equilibrium in the preparation of polyol esters.According to the procedure known from DE 199 40 991 A1, thewater-immiscible solvent which acts as an entraining agent and whichshould have a boiling point of less than 112° C. is added to thereaction mixture only on attainment of a temperature of at least 140° C.

The crude ester obtained after removal of the water of reaction and ofexcess unconverted starting materials, appropriately the carboxylic acidadded in excess, can first be treated with an alkaline reagent, forexample with an aqueous sodium carbonate or sodium hydroxide solution,in order to remove last residues of acidic constituents. After washingwith water and treatment with bleaching earth and activated carbon, lasttraces of coloring and odorous substances can be removed by applyingreduced pressure at elevated temperature.

Processes for working up crude polyol esters are known, for example,from U.S. Pat. No. 2,469,446 A1. In some cases, the treatment withbleaching agents and activated carbon has to be repeated more than oncein order to obtain end products with satisfactory color properties.

For the workup of the crude ester, U.S. Pat. No. 5,324,853 A1 proposesremoving the excess acid by passing through nitrogen or steam, adding anadsorbent, neutralizing residual organic acid with a base and filteringoff solids obtained. The residual amounts of acid present in thefiltrate are removed with the passage of steam or nitrogen withsimultaneous application of a reduced pressure and recycled back intothe esterification reaction. Solids obtained in the vacuum treatment areremoved in a final fine filtration. According to the procedure knownfrom DE 199 40 991 A1, the crude ester is dried after alkali treatment,for example by passing an inert gas through the product or applyingreduced pressure and optionally additionally distilling under reducedpressure. To improve the color of polyol esters, WO 94/18153 A1 proposesa subsequent treatment with an aqueous hydrogen peroxide solution.

Owing to the quality criteria described at the outset for polyol esters,the process steps in the esterification stage with removal of the waterof reaction and in the workup of the crude ester are very importantprocess features, since the adjustment of these process stepssignificantly influences the sensory and optical properties of the endproducts. More particularly, high demands are made on the colorproperties, such as low color number and high color stability, of thepolyol esters. The structure of the starting materials, the polyhydricalcohols and the acids, is, in contrast, crucial for the mechanical andthermal properties of the polymer materials plasticized with the polyolesters and influences the hydrolysis and oxidation stability oflubricants.

In the course of preparation of polyol esters, the carboxylic acid usedin excess, for example an aliphatic monocarboxylic acid, is removed inthe course of the workup process and recycled back into theesterification process. In the continuous process regime, the recyclingis effected during the running process, whereas, in the batchwiseprocess, the excess aliphatic monocarboxylic acid removed is firstcollected and reused in the next batch. For the economic viability ofthe esterification process, high reuse rates of the aliphaticmonocarboxylic acid are desirable. However, this is opposed by the factthat, with increasing reuse, the acid quality suffers as a result of theformation and concentration of by-products, such that the aliphaticmonocarboxylic acid recovered has to be at least partly finallydischarged and replaced by fresh acid in the course of continuous orbatchwise operation. For an economically viable process, it is, however,desirable to use the aliphatic monocarboxylic acid recovered asfrequently as possible in the continuous esterification process or, inthe case of batchwise operation, in the subsequent production batches,without the quality of the desired polyol ester suffering.

SUMMARY OF INVENTION

It has now been found that, surprisingly, polyol esters can be preparedfrom polyols and linear or branched aliphatic monocarboxylic acids withan excellent color number and color stability when the aliphaticmonocarboxylic acid recovered in the course of workup of the crudeesterification mixture is not recycled fully, but only partly, back intothe esterification reaction.

The invention therefore consists in a process for preparing polyolesters by reacting polyols with linear or branched aliphaticmonocarboxylic acids having 3 to 20 carbon atoms and then working up thereaction mixture by means of steam treatment. The process ischaracterized in that in a first fraction the aliphatic monocarboxylicacid removed during the reaction is removed fully or partly from theprocess, in a second fraction the aliphatic monocarboxylic acid stillpresent in the reaction mixture after reaction has ended is removed andrecycled fully back into the esterification reaction, and in a thirdfraction the residual amount of aliphatic monocarboxylic acid removed inthe course of steam treatment of the reaction product is removed fullyfrom the process.

The controlled discharge and recycling of the aliphatic monocarboxylicacid obtained during the reaction and subsequent workup phase allowscoloring components which form during the esterification reaction to beremoved from the process in a simple manner. It has been found that,surprisingly, coloring components can be removed in a first steptogether with the mixture of water of reaction and aliphaticmonocarboxylic acid removed during the esterification reaction. Coloringcomponents in the residual amounts of the aliphatic monocarboxylic acidwhich are obtained in the course of the workup process in the steamtreatment are also accumulated in a third fraction. The aliphaticmonocarboxylic acid obtained in the second fraction, which is removedfrom the crude polyol ester after the esterification reaction, iscontaminated only with small amounts of coloring components and can berecycled fully back into the esterification process. In the case of abatchwise process regime, this means that this acid fraction can bereused in subsequent reaction batches.

The inventive procedure can be considered as a fractional distillationof the aliphatic monocarboxylic acid out of the reaction mixture and outof the crude ester obtained. In a first fraction, which may also bereferred to as the first runnings fraction, the coloring componentsaccumulate in the mixture of aliphatic monocarboxylic acid and water ofreaction, which is removed during the esterification reaction. In asecond fraction, which can also be referred to as the intermediatefraction, virtually pure aliphatic monocarboxylic acid is obtained fromthe crude ester after the esterification reaction has ended, which iscontaminated only with a low level of coloring components. In a thirdfraction, which can also be referred to as the tailings fraction,contaminated aliphatic monocarboxylic acid is again obtained in thecourse of the steam treatment.

When the process according to the invention is performed batchwise, thealiphatic monocarboxylic acid which is contaminated with coloringcomponents and is obtained as the first runnings fraction in the firstfraction and as the tailings fraction in the third fraction is collectedand not used for the subsequent production batches. In the case of thecontinuous process regime, these fractions are discharged continuouslyand replaced by fresh acid as in the batchwise method or by the lesscontaminated intermediate fraction from preceding batches. These removedfractions can be collected, purified in a separate distillation step andthen used again in the esterification process. The intermediate fractionwith a low level of contamination by coloring components or the secondfraction is used for the next production batch in the case of thebatchwise process regime, or recycled directly into the esterificationreactor in the case of the continuous process regime.

The inventive measure of fractional removal and recycling of the excessaliphatic monocarboxylic acid concentrates the coloring components in asmall portion of the excess aliphatic monocarboxylic acid used, whilethe greater residual amount of the aliphatic monocarboxylic acid issubstantially free of coloring components. This intermediate fractionwith a low level of contamination can be recycled into theesterification process significantly more frequently than all of thecontaminated excess aliphatic monocarboxylic acid removed, in which thecoloring components are present diluted in a relatively large amount ofacid, without the color quality of the desired polyol ester sufferingtoo much. The removal of a small portion of highly contaminatedaliphatic monocarboxylic acid and the replacement to the required acidexcess by addition of fresh acid or intermediate fractions with a lowlevel of contamination from preceding batches utilizes the aliphaticmonocarboxylic acid used to a significantly more productive degree thana procedure in which all of the contaminated aliphatic monocarboxylicacid removed is used again for the esterification reaction withoutreplacement of acid. In this procedure, the entire amount of acid has tobe removed from the process and replaced by fresh acid after only a fewreuses, in order to obtain polyol ester with adequate color number andcolor stability.

The reaction between polyol and aliphatic monocarboxylic acid, dependingon the starting materials, sets in within the range from about 120 to180° C. and can be conducted to completion in different ways.

Further features and advantages will become apparent from the discussionwhich follows.

DETAILED DESCRIPTION

One configuration of the process according to the invention firstinvolves heating proceeding from room temperature to a temperature up toa maximum of 280° C., preferably up to 250° C., and, with thetemperature kept constant, lowering the pressure in stages proceedingfrom standard pressure, in order to facilitate the removal of the waterof reaction. The selection of the pressure stages, whether one, two ormore than two stages, and of the pressure to be established at theparticular stage, may be varied over a wide range and adjusted to theparticular conditions. For example, in a first stage, the pressure canbe lowered proceeding from standard pressure first down to 600 hPa, andthen the reaction can be conducted to completion at a pressure of 300hPa. These pressure figures are guide values which are appropriatelycomplied with.

In addition to the variation of the pressure, it is likewise alsopossible to alter the temperature proceeding from room temperature inone, two or more than two stages during the esterification reaction,such that, at constant pressure, the temperature is increased from stageto stage, typically up to a maximum temperature of 280° C. However, ithas been found to be appropriate to heat to a maximum of 280° C. withthe temperature rising from stage to stage, and also to lower thepressure from stage to stage. For example, the esterification reactioncan be conducted proceeding from room temperature in a first stage at atemperature up to 190° C. A reduced pressure down to 600 hPa is likewiseapplied, in order to accelerate the driving-out of the water ofreaction. On attainment of the temperature stage of 190° C., thepressure is lowered once again down to 300 hPa, and the esterificationreaction is conducted to completion at a temperature up to 250° C. Thesetemperature and pressure figures are guide values which areappropriately complied with. The temperature and pressure conditions tobe established at the particular stages, the number of stages and theparticular temperature increase or pressure reduction rate per unit timecan be varied over a wide range and adjusted in accordance with thephysical properties of the starting compounds and of the reactionproducts, the temperature and pressure conditions of the first stagebeing established proceeding from standard pressure and roomtemperature. It has been found to be particularly appropriate toincrease the temperature in two stages and to lower the pressure in twostages.

The lower limit of the pressure to be established depends on thephysical properties, such as boiling points and vapor pressures, of thestarting compounds and of the reaction products formed, and is alsodetermined by the available plant apparatus. Proceeding from standardpressure, it is possible to work in stages within these limits, withpressures decreasing from stage to stage. The upper temperature limit,typically 280° C., should be complied with in order to prevent theformation of decomposition products, which adversely affect color amongother properties. The lower limit of the temperature stages isdetermined by the reaction rate, which must still be sufficiently highto complete the esterification reaction within an acceptable time.Within these limits, it is possible to work in stages with temperaturesrising from stage to stage.

The water of reaction formed is distilled out of the reaction vessel inthe course of the reaction together with the excess monocarboxylic acidand passed into a downstream phase separator in which the monocarboxylicacid and water separate according to their solubility properties. Themonocarboxylic acid used may also form an azeotrope with water under thereaction conditions and be capable of removing the water of reaction asan entraining agent. The progress of the reaction can be monitored bythe water obtained. The water which separates out is removed from theprocess, while the monocarboxylic acid from the phase separator flowsback into the reaction vessel. The addition of a further organicsolvent, such as hexane, 1-hexene, cyclohexane, toluene, xylene orxylene isomer mixtures, which assumes the task of the azeotroping agent,is not ruled out, but restricted to a few exceptional cases. Theazeotroping agent can be added as early as the start of theesterification reaction or on attainment of relatively hightemperatures. When the theoretical amount of water expected has beenobtained or the hydroxyl number, for example determined according to DIN53240, has fallen below a fixed value, the reaction is ended by allowingthe reaction mixture to cool.

The aliphatic monocarboxylic acid which accumulates in the phaseseparator after the reaction has ended can be referred to as the firstfraction and is contaminated with coloring components. This fraction isremoved from the process and purified in a separate acid distillation.

The reaction mixture obtained after the reaction has ended comprises, aswell as the polyol ester as the desired reaction product, excess andunconverted aliphatic monocarboxylic acid. The latter is distilled outof the crude product, appropriately with application of a reducedpressure. The distillation conditions to be established appropriatelyare determined by the physical properties of the polyol used and of thealiphatic monocarboxylic acid, and can be determined by simplepreliminary tests. For example, temperatures up to 200° C. and pressuresless than 300 hPa, preferably less than 50 hPa, are employed. Thealiphatic monocarboxylic acid obtained can be considered as the secondfraction or intermediate fraction. It is contaminated only with a lowlevel of coloring components and can therefore be recycled directly backinto the esterification process or collected as a starting material fora further batch.

The reaction of polyols and aliphatic monocarboxylic acids can beperformed without use of a catalyst. This variant of the reaction hasthe advantage that it avoids adding extraneous substances to thereaction mixture, which can lead to undesired contamination of thepolyol ester. However, it is generally necessary in that case tomaintain higher reaction temperatures because only in this way is itensured that the reaction proceeds at a sufficient, i.e. economicallyacceptable, rate. In this context, it should be noted that the increasein the temperature can lead to thermal damage to the polyol ester. It istherefore not always possible to avoid the use of a catalyst whichfacilitates the reaction and increases the reaction rate. Frequently,the catalyst may be an excess of the aliphatic monocarboxylic acid,which is simultaneously a reaction component of the polyol, such thatthe reaction proceeds autocatalytically. Otherwise, the customaryesterification catalysts are suitable for influencing the reaction rate,such as sulphuric acid, formic acid, polyphosphoric acid,methanesulfonic acid or p-toluenesulfonic acid, and likewisecombinations of such acids. It is likewise possible to use metalliccatalysts, such as titanium-, zirconium- or tin-containing catalysts,for example the corresponding alkoxides or carboxylates. It is alsopossible to use catalytically active compounds which are insoluble inthe reaction system and solid under reaction conditions, such as alkalimetal or alkaline earth metal hydrogensulfate, for example sodiumhydrogensulfate. Solid catalysts are removed from the reaction mixtureafter the esterification has ended by simple filtration together withany adsorbent present. The amount of the catalyst used can extend over awide range. It is possible to use 0.001% by weight up to 5% by weight ofcatalyst, based on the reaction mixture. Since greater amounts ofcatalysts, however, give barely any advantages, the catalystconcentration is typically 0.001 to 1.0% and preferably 0.01 to 0.5% byweight, based in each case on the reaction mixture. Appropriately, adecision is made for each individual case, optionally by preliminarytests, as to whether no catalyst should be employed at relatively hightemperature or a catalyst should be employed at relatively lowtemperature.

The polyol is allowed to react with excess aliphatic monocarboxylicacid, typically without addition of a catalyst, such that the excessaliphatic monocarboxylic acid itself acts as a catalyst. The aliphaticmonocarboxylic acid used in excess can also serve as an entraining agentfor the water of reaction released, and the water/acid mixture removedis likewise capable of entraining the coloring components. Themonocarboxylic acid generally has a lower boiling point than the polyolused and can therefore be removed from the crude ester by distillationin a simple manner. The aliphatic monocarboxylic acid is used in a 10 to50% molar and preferably in a 20 to 40% molar excess per mole ofhydroxyl group to be esterified in the polyol.

In a further configuration of the process according to the invention,the esterification is performed in the presence of an adsorbent. Thisinvolves using porous, large-surface area solid materials which aretypically used in chemical practice both in laboratory and in industrialplants. Examples of such materials are high-surface area polysilicicacids such as silica gels (silica xerogels), kieselguhr, high-surfacearea aluminum oxides and aluminum oxide hydrates, mineral materials suchas clays, carbonates or activated carbon. Activated carbon has beenfound to be particularly useful. In general, the adsorbent is suspendedin finely divided form in the reaction solution, which is agitated byintensive stirring or by introducing an inert gas. This achievesintimate contact between the liquid phase and the adsorbent. The massratio of the liquid phase to adsorbent can be adjusted substantiallyfreely and hence according to the individual requirements. It has beenfound to be useful to use 0.05 to 30, preferably 0.1 to 5 and especially0.1 to 1 parts by weight of adsorbent per 100 parts by weight of liquidphase. After the reaction has ended, the adsorbent can be removed fromthe process and recycled into the esterification vessel and reused.Reuse is possible until the decolorizing power of the adsorbent isexhausted. However, it is also possible to leave the adsorbent in thecrude product and to remove it at any convenient stage during the workupprocess.

It is likewise possible, during the esterification reaction, to pass aninert gas, for example nitrogen, carbon dioxide or the noble gases,through the reaction mixture in order to expel the water of reaction.

After removing the excess aliphatic monocarboxylic acid as theintermediate fraction, the crude ester obtained is subjected to atreatment with steam, which, for example, can be effected in simple formby introducing steam into the crude product. One advantage of the steamtreatment is that catalyst still present in the course thereof isdestroyed and converted to hydrolysis products which can be filtered offefficiently. If the esterification reaction is performed in the presenceof an adsorbent, the adsorbent which is already present facilitates theseparation of the catalyst conversion products. Otherwise, it may befound to be advantageous to add the adsorbent at the start of the steamtreatment. The presence of an adsorbent during the steam treatmentlikewise has an advantageous effect on the color and on the colorstability of the polyol ester. However, it is also possible to filteroff the adsorbent after the esterification reaction has ended and excessstarting compounds have been removed, i.e. before performance of thesteam distillation.

The steam treatment is generally performed at standard pressure,although the employment of a slightly reduced pressure, appropriatelydown to 400 hPa, is not ruled out. The steam treatment is effectedgenerally at temperatures of 100 to 250° C., preferably of 150 to 220°C. and especially of 170 to 200° C., and is also guided by the physicalproperties of the polyol esters to be prepared in each case.

In the process step of steam treatment, it is found to be appropriate toproceed in a very gentle manner during the heating period until theattainment of the working temperature, in order to heat the crude esterto the required temperature of the steam treatment.

The duration of the steam treatment can be determined by routine testsand it is generally performed over a period of 0.5 to 5 hours. Too longa steam treatment leads to an undesired increase in the color number ofthe polyol ester and should therefore be avoided. An enhanceddegradation reaction of the polyol ester to acidic compounds is alsoobserved, the content of which is manifested in a rise in theneutralization number or acid number, for example determined accordingto DIN EN ISO 3682/ASTM D 1613. In addition, in the case of polyolesters based on other diols, for example triethylene glycol ortetraethylene glycol, undesired degradation of the ether chain may setin. In the case of too short a treatment time, the removal of residualacid and water is not sufficiently effective and the desired polyolester still has too high an undesired acid number and too high a watercontent. In the case of excessively short treatment time too, only aminor advantageous effect is observed on the color number of the polyolester.

The residues of aliphatic monocarboxylic acid removed in the course ofsteam distillation, which can also be considered as the third fractionor as the tailings fraction, are again contaminated with coloringcomponents and are discharged from the process. This fraction can bepurified separately from or together with the aliphatic monocarboxylicacid removed in the first fraction or as the first runnings fraction ina separate distillation stage, and recycled back into the esterificationprocess.

The steam treatment is followed, optionally after filtration of theadsorbent and other solids obtained, by the drying of the polyol ester,for example by passing an inert gas through the product at elevatedtemperature. It is also possible to simultaneously apply a reducedpressure at elevated temperature and optionally to pass an inert gasthrough the product. Even without the action of an inert gas, it ispossible to work only at elevated temperature or only under reducedpressure. The particular drying conditions, such as temperature,pressure and duration, can be determined by simple preliminary tests. Ingeneral, temperatures in the range from 80 to 250° C. and preferably 100to 180° C., and pressures of 0.2 to 500 hPa, preferably 1 to 200 hPa andespecially 1 to 20 hPa, are employed. Thereafter, the crude ester isfiltered, if this has not already been done, in order to free it of thesolids, the hydrolysis products of the catalyst and the adsorbent, ifadded in the esterification stage or before the steam treatment. Thefiltration is effected in conventional filtering apparatus at standardtemperature or at temperatures up to 120° C. The filtration can besupported by common filtering aids such as cellulose, silica gel,kieselguhr, or wood flour. However, the use thereof is restricted toexceptional cases.

The measure of fractional removal of the unconverted and excessaliphatic monocarboxylic acid allows the coloring components to beremoved from the process in a simple manner, such that the amount ofadsorbents which is added to lighten the color in some cases can bereduced.

After the steam treatment or after the drying, a further treatment ofthe polyol ester with oxidizing agents, for example with an aqueoushydrogen peroxide solution or with ozone or ozone-containing gases, mayalso be provided instead of a treatment with adsorbents, for examplewith activated carbon, in order to improve the color number. However,this measure is restricted only to a few exceptional cases. The use ofan aqueous hydrogen peroxide solution or of ozone or ozone-containinggases, however, has the advantage over the use of solid adsorbents thatadditional filtration steps become dispensable.

The aliphatic monocarboxylic acid with only a low level of contaminationobtained as the intermediate fraction can be reused more frequently,without this resulting in reductions in the color quality of the desiredpolyol. When, in contrast, all fractions of aliphatic monocarboxylicacid obtained are combined and recycled into the process, only limitedreuse is possible owing to the rising color number of the polyol ester.When, therefore, the color-contaminated acid fractions are removedduring the esterification reaction and during the workup according tothe inventive procedure, the aliphatic monocarboxylic acid recovered asthe second fraction or as the intermediate fraction can be exploitedmore productively.

In order to establish the desired acid excess based on polyol in theesterification reaction, the intermediate fractions can be collected andrecycled back into the esterification process, optionally after furtheraddition of fresh acid, or be used as a starting material for subsequentbatches.

Light-colored polyol esters are obtained, which also satisfy theremaining specifications, such as water content, residual acid content,residual content of catalyst constituents and residual content ofmonoester.

The polyhydric alcohols or polyols used as starting materials for theprocess according to the invention satisfy the general formula (I)

R(OH)_(n)  (I)

in which R is an aliphatic or cycloaliphatic hydrocarbon radical having2 to 20 and preferably 2 to 10 carbon atoms, and n is an integer of 2 to8, preferably 2, 3, 4, 5 or 6.

Suitable polyols are likewise compounds of the general formula (II)

H—(—O—[—CR¹R²—]_(m)—)_(o)—OH  (II)

in which R¹ and R² are each independently hydrogen, an alkyl radicalhaving 1 to 5 carbon atoms, preferably methyl, ethyl or propyl, or ahydroxyalkyl radical having 1 to 5 carbon atoms, preferably thehydroxymethyl radical, m is an integer of 1 to 10, preferably 1 to 8 andespecially 1, 2, 3 or 4, o is an integer of 2 to 15, preferably 2 to 8and especially 2, 3, 4 or 5.

Suitable polyols which can be converted by the process according to theinvention to light-colored polyol esters are, for example,1,3-propanediol, 1,3-butanediol, 1,4-butanediol, neopentyl glycol,2,2-dimethylolbutane, trimethylolethane, trimethylolpropane,ditrimethylolpropane, trimethylolbutane,2,2,4-trimethylpentane-1,3-diol, 1,2-hexanediol, 1,6-hexanediol,pentaerythritol or dipentaerythritol or3(4),8(9)-dihydroxymethyltricyclo[5.2.1.0^(2,6)]decane.

Useful further polyols include ethylene glycol and 1,2-propylene glycol,and the oligomers thereof, especially di-, tri- and tetraethylene glycolor dipropylene glycol, tripropylene glycol or tetrapropylene glycol.Ethylene and propylene glycols are industrially produced chemicals. Thebase substance for preparation thereof is ethylene oxide and propyleneoxide, from which 1,2-ethylene glycol and 1,2-propylene glycol areobtained by heating with water under pressure. Diethylene glycol isobtained by ethoxylation from ethylene glycol. Triethylene glycol isobtained, like tetraethylene glycol, as a by-product in the hydrolysisof ethylene oxide to prepare ethylene glycol. Both compounds can also besynthesized by reacting ethylene glycol with ethylene oxide. Dipropyleneglycol, tripropylene glycol, tetrapropylene glycol and higherpropoxylation products are obtainable from the multiple addition ofpropylene oxide onto 1,2-propylene glycol.

To obtain light-colored polyol esters by the process according to theinvention, linear or branched, aliphatic monocarboxylic acids having 3to 20 carbon atoms in the molecule are used. Even though preference isgiven to saturated acids in many cases, depending on the particularfield of use of the plasticizers or lubricants, it is also possible touse unsaturated carboxylic acids as a reaction component for estersynthesis. Examples of monocarboxylic acids as components of polyolesters are propionic acid, n-butyric acid, isobutyric acid, n-pentanoicacid, 2-methylbutyric acid, 3-methylbutyric acid, 2-methylpentanoicacid, n-hexanoic acid, 2-ethylbutyric acid, n-heptanoic acid,2-methylhexanoic acid, cyclohexanecarboxylic acid, 2-ethylhexanoic acid,n-nonanoic acid, 2-methyloctanoic acid, isononanoic acid,3,5,5-trimethylhexanoic acid, 2-propylheptanoic acid,2-methyl-undecanoic acid, isoundecanecarboxylic acid,tricyclodecanecarboxylic acid and isotridecanecarboxylic acid. The novelprocess has been found to be particularly useful for the preparation ofpolyol esters of monoethylene glycol, or of the oligomeric ethyleneglycols and of 1,2-propylene glycol, or of the oligomeric propyleneglycols with C₄- to C₁₃- or C₅- to C₁₀-monocarboxylic acids, and forpreparation of polyol esters based on 1,3-butanediol, neopentyl glycol,2,2,4-trimethylpentane-1,3-diol, trimethylolpropane,ditrimethylolpropane, pentaerythritol or3(4),8(9)-dihydroxymethyltricyclo[5.2.1.0^(2,6)]decane.

The polyol esters of ethylene glycol and the oligomers thereof areoutstandingly suitable as plasticizers for all common high molecularweight thermoplastic substances. They have been found to be particularlyuseful as an additive to polyvinyl butyral which is used admixed withglycol esters as an intermediate layer for production of multilayer orcomposite glasses. They can likewise be used as coalescence agents orfilm-forming assistants in aqueous dispersions of polymers which findvarious uses as coating materials. The preparation process according tothe invention makes it possible to prepare, in a simple manner, polyolesters with outstanding color properties which also satisfy furtherquality demands, such as low odor or a low acid number. The processaccording to the invention is particularly suitable for preparingtriethylene glycol di-2-ethylhexanoate (3G8 Ester), tetraethylene glycoldi-n-heptanoate (4G7 Ester), triethylene glycol di-2-ethylbutyrate (3G6Ester), triethylene glycol di-n-heptanoate (3G7 Ester) or tetraethyleneglycol di-2-ethylhexanoate (4G8 Ester).

The process according to the invention can be performed continuously orbatchwise in the reaction apparatus typical for chemical technology.Useful apparatus has been found to be stirred tanks or reaction tubes,the batchwise reaction regime being preferred.

The process according to the invention is illustrated in detail in theexamples which follow, but it is not restricted to the embodimentdescribed.

WORKING EXAMPLES Example 1

Preparation of triethylene glycol di-2-ethylhexanoate (3G8 Ester);esterification in the presence of activated carbon with fresh acid

The esterification of triethylene glycol with 2-ethylhexanoic acid wasperformed in a heatable 1 l four-neck flask provided with stirrer,internal thermometer and a water separator.

The flask was initially charged with 250 grams (1.66 mol) of triethyleneglycol and 680 grams (4.72 mol) of fresh 2-ethylhexanoic acid, and also0.4% by weight of activated carbon, based on the overall reactionmixture. While stirring and applying a slightly reduced pressure of 900hPa, the mixture was heated to 225° C. On attainment of thistemperature, the pressure was reduced stepwise to 400 hPa, and water ofreaction formed was removed on the water separator, while2-ethylhexanoic acid flowed back into the reaction vessel. The course ofthe reaction was monitored by continuously weighing the water dischargedvia the water separator and by the course of the hydroxyl number. Aftera total of 14.5 hours of reaction time, the reaction was ended at aresidual hydroxyl number of 4.2 mg KOH/g (according to DIN 53240).

In the water separator, after the esterification reaction had ended,13.3 g of 2-ethylhexanoic acid were obtained as the first fraction. Thisfraction was not used for the next esterification batch.

Subsequently, the excess 2-ethylhexanoic acid was distilled off at atemperature of 200° C. and at a pressure of 20 hPa over a period of 3.75hours. 187.1 g of 2-ethylhexanoic acid were obtained as the intermediatefraction, which was reusable for subsequent esterification batches.

There followed a steam distillation at 200° C. and at standard pressureover a period of 2.5 hours. In addition to the amount of water obtained,as the third fraction, a residual amount of 2-ethylhexanoic acid of 0.7g was also recovered, which was likewise not used for subsequentesterification batches.

After final filtration to remove the activated carbon, light-coloredtriethylene glycol di-2-ethylhexanoate with the color number reported inTable 1 was obtained.

Example 2

Preparation of triethylene glycol di-2-ethylhexanoate (3G8 Ester);esterification in the presence of activated carbon, reuse of theintermediate fraction from preceding esterification batches

Example 2 was performed analogously to example 1, with the soleexception that, instead of fresh 2-ethylhexanoic acid, the2-ethylhexanoic acid intermediate fraction collected from precedingesterification batches was used.

Example 3 Comparative Example

Preparation of triethylene glycol di-2-ethylhexanoate (3G8 Ester);esterification in the presence of activated carbon, reuse of all of thereturn acid from preceding esterification batches.

Example 3 was performed analogously to example 2, with the soleexception that, instead of the 2-ethylhexanoic acid intermediatefraction, all of the 2-ethylhexanoic acid recovered from the precedingesterification batches was used.

The color numbers of the worked-up triethylene glycoldi-2-ethylhexanoate esters obtained according to examples 1 to 3 arelisted in table 1 below. The ester contents determined by gaschromatography and the other indices, such as residual acid content orwater content, were in agreement.

TABLE 1 Color numbers of triethylene glycol di-2-ethylhexanoate,prepared according to Examples 1, 2 and 3 Example 1 Example 2 Example 3Hazen color number 24 15 45 (DIN ISO 6271)

The inventive measure of recycling the aliphatic monocarboxylic acidrecovered in the esterification partially into the esterification stageor using it partially for subsequent esterification batches allowspolyol esters to be obtained with outstanding color number, whichenables the use thereof in a multitude of applications.

While the invention has been described in detail, modifications withinthe spirit and scope of the invention will be readily apparent to thoseof skill in the art. In view of the foregoing discussion, relevantknowledge in the art and references discussed above, the disclosures ofwhich are all incorporated herein by reference, further description isdeemed unnecessary. In addition, it should be understood that aspects ofthe invention and portions of various embodiments may be combined orinterchanged either in whole or in part. Furthermore, those of ordinaryskill in the art will appreciate that the foregoing description is byway of example only, and is not intended to limit the invention.

1. Process for preparing polyol esters by reacting polyols with linearor branched aliphatic monocarboxylic acids having 3 to 20 carbon atomsand then working up the reaction mixture by means of steam treatment,characterized in that in a first fraction the aliphatic monocarboxylicacid removed during the reaction is removed fully or partly from theprocess, in a second fraction the aliphatic monocarboxylic acid stillpresent in the reaction mixture after reaction has ended is removed andrecycled fully back into the esterification reaction, and in a thirdfraction the residual amount of aliphatic monocarboxylic acid removed inthe course of steam treatment of the reaction product is removed fullyfrom the process.
 2. Process according to claim 1, characterized in thatthe reaction of the starting compounds involves heating to a temperatureup to a maximum of 280° C., preferably up to 250° C., and lowering thepressure from stage to stage with the temperature kept constant. 3.Process according to claim 1, characterized in that the reaction of thestarting compounds involves heating at constant pressure from stage tostage up to a maximum temperature of 280° C.
 4. Process according toclaim 1, characterized in that the reaction of the starting compoundsinvolves heating at a temperature rising from stage to stage to amaximum of 280° C., and also lowering the pressure from stage to stage.5. Process according to claim 4, characterized in that the reaction ofthe starting compounds involves allowing them to react in a first stageat a temperature up to 190° C. and at a pressure up to 600 hPa, andconducting the reaction to completion in a second stage by increasingthe temperature up to 250° C. and at a pressure up to 300 hPa. 6.Process according to claim 1, characterized in that the reaction of thestarting compounds is performed in the presence of a catalyst. 7.Process according to claim 6, characterized in that the catalysts usedare catalysts comprising titanium, zirconium or tin.
 8. Processaccording to claim 1, characterized in that the reaction of the startingcompounds is performed in the presence of an adsorbent.
 9. Processaccording to claim 8, characterized in that the adsorbent is used in anamount of 0.05 to 30, preferably of 0.1 to 5.0 and especially of 0.1 to1.0 parts by weight per 100 parts by weight of liquid phase.
 10. Processaccording to claim 8, characterized in that the adsorbent used is silicagel, kieselguhr, aluminum oxide, aluminum oxide hydrates, clays,carbonates or activated carbon.
 11. Process according to claim 1,characterized in that the reaction of the starting compounds isperformed in the presence of an inert gas.
 12. Process according toclaim 1, characterized in that the steam treatment is performed at atemperature of 100 to 250° C., preferably of 150 to 220° C. andespecially of 170 to 200° C.
 13. Process according to claim 1,characterized in that the polyol ester, after the steam treatment, isdried at temperatures of 80 to 250° C., preferably 100 to 180° C., andat pressures of 0.2 to 500 hPa, preferably 1 to 200 hPa and especiallyof 1 to 20 hPa.
 14. Process according to claim 13, characterized in thatthe polyol ester is dried in the presence of an inert gas.
 15. Processaccording to claim 1, characterized in that the polyol ester is filteredafter the steam treatment.
 16. Process according to claim 13,characterized in that the polyol ester is filtered after the drying. 17.Process according to claim 12, characterized in that the polyol ester istreated with an oxidizing agent after the steam treatment or after thedrying.
 18. Process according to claim 17, characterized in that theoxidizing agents used are hydrogen peroxide, ozone or ozone-containinggases.
 19. Process according to claim 1, characterized in that thepolyols used are compounds of the general formula (I)R(OH)_(n)  (I) in which R is an aliphatic or cycloaliphatic hydrocarbonradical having 2 to 20 and preferably 2 to 10 carbon atoms, and n is aninteger of 2 to 8, preferably 2, 3, 4, 5 or
 6. 20. Process according toclaim 1, characterized in that the polyols used are compounds of thegeneral formula (II)H—(—O—[—CR¹R²—]_(m)—)_(o)—OH  (II) in which R¹ and R² are eachindependently hydrogen, an alkyl radical having 1 to 5 carbon atoms,preferably methyl, ethyl or propyl, or a hydroxyalkyl radical having 1to 5 carbon atoms, preferably the hydroxymethyl radical, m is an integerof 1 to 10, preferably 1 to 8 and especially 1, 2, 3 or 4, o is aninteger of 2 to 15, preferably 2 to 8 and especially 2, 3, 4 or
 5. 21.Process according to claim 19, characterized in that the polyols usedare 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol,neopentyl glycol, 2,2-dimethylolbutane, trimethylolethane,trimethylolpropane, trimethylolbutane, 2,2,4-trimethylpentane-1,3-diol,1,2-hexanediol, 1,6-hexanediol, pentaerythritol, ethylene glycol or3(4),8(9)-dihydroxymethyltricyclo[5.2.1.0^(2,6)]-decane.
 22. Processaccording to claim 20, characterized in that the polyols used areditrimethylolpropane, dipentaerythritol, diethylene glycol, triethyleneglycol, tetraethylene glycol, dipropylene glycol, tripropylene glycol ortetrapropylene glycol.
 23. Process according to claim 1, characterizedin that the aliphatic monocarboxylic acid converted is propionic acid,n-butyric acid, isobutyric acid, n-pentanoic acid, 2-methylbutyric acid,3-methylbutyric acid, 2-methylpentanoic acid, n-hexanoic acid,2-ethylbutyric acid, n-heptanoic acid, 2-methylhexanoic acid,2-ethylhexanoic acid, n-nonanoic acid, 2-methyloctanoic acid,isononanoic acid, 3,5,5-trimethylhexanoic acid or 2-propylheptanoicacid.
 24. Process according to claim 1 for preparing triethylene glycoldi-2-ethylhexanoate, tetraethylene glycol di-n-heptanoate, triethyleneglycol di-2-ethylbutyrate, triethylene glycol di-n-heptanoate ortetraethylene glycol di-2-ethylhexanoate.